The present invention relates to methods and apparatus for molding homogeneous articles from amorphous and crystalline thermoplastic resins, and, more particularly, to methods and apparatus which maintain high temperature for a period of time sufficient to relax a resin at its surface by providing a booster layer and to eliminate stress variations in the molded part without the need for adding and removing heat, multiple stations or additional processes such as annealing.
In almost all known molding operations, pressure and heat are applied to cause a plastic to flow into a desired shape. The shape is then fixed by cooling. Heating, flow under pressure and cooling under pressure are required in this sequence. Injection, compression, and blow molding are commonly employed production processes for molding large numbers of articles with reusable molds.
Injection molding is an automated process that is used to rapidly produce plastic molded articles. Compact audio discs, for example, are molded at a rate of 400 to 600 discs per hour from a single mold. The injection molding process has the following basic steps:
Step (1) Circulate fluid through the mold to bring it to the desired temperature.
Step (2) Close the mold.
Step (3) Force a molten thermoplastic resin through a small opening called a gate into the mold cavity that defines the molded article geometry until the cavity is filled.
Step (4) Cool the molded article until it is rigid enough to be removed from the mold.
Step (5) Open the mold and remove the molded article.
In many cases additional molten thermoplastic resin is packed into the mold cavity after Step (3) to compensate for the loss of resin volume as it shrinks during the cooling period. In other cases, the resin is compressed to compensate for the shrinkage that occurs after the gate is shut off. The gate may be shut off immediately after the cavity is filled.
The injection molding process has many advantages, but it also has several inherent problems, particularly regarding high precision molded articles that must have uniform properties throughout. High shear flow stresses are created in the molded article during injection. When the hot thermoplastic resin melt contacts the much colder mold cavity surface, a thin frozen region starts immediately to form. The plastic then continues to cool from the cold mold surface toward the center of the molded article. An intermediate layer is formed when the molecules in the melt contact the outer frozen region and freeze to it. Resulting high flow stresses cause these molecules to become oriented in the direction of flow. The intermediate layer is highly shear stressed. The outer and the intermediate layers together form a region referred to as `skin.` The stresses are frozen in the skin and cannot relax. The thickness of the skin increases as the square root of resin contact time with the cold mold surface.
The time required for the molecular stresses to relax increases with decreasing temperature. For amorphous plastics, stress relaxation is almost impossible when the plastic is below its glass transition temperature. The central core of plastic melt flowing into the cavity is thermally insulated by the skin region. The core remains hot for a longer time than the skin, thereby providing the time it must be at an elevated temperature in order for reorientation of molecules in the core region to occur. The core region consequently has little flow-induced orientation and shear stress. The orientations and layers can be seen in FIGS. 1 to 3, which are taken from "The Interrelationship of Flow, Structure, And Properties In Injection Molding" by L. R. Schmidt, Corp. R&D, G.E., Schenectady, New York. In FIG. 1 a rectangular mold cavity 1 is shown being filled with molten plastic 2 through a gate 3. The hydrodynamic skin-core structure in the mold cavity 1 during filling is shown in FIG. 2, wherein the skin 4 of the plastic melt within the mold cavity at the top and bottom surfaces thereof thermally insulates the central core 5 of flowing plastic melt during filling of the mold. As shown in FIG. 3, the skin is formed of an outer layer, identified as zone I, and an intermediate layer which is highly oriented and highly shear stressed, identified as zone II. The core zone is identified as zone III in FIG. 3.
Under typical injection molding conditions, flow rates vary significantly throughout the cavity resulting in variable skin thickness. Mechanical and physical properties of the molded article vary considerably depending on the distribution of skin and core thicknesses as well as the degree of orientation and crystallinity at different locations in the molded article.
During flow, higher pressures in the core region cause higher shear stresses and molecular orientation in the intermediate layer as the core material is pushed through. When the cavity is completely full, pressure increases rapidly. High pressure is then maintained as the molded article continues to cool and the thickness of the frozen layer increases. The volume of the plastic decreases as it solidifies and the injection pressure packs in more plastic until the gate closes or pressure is removed. After the gate closes, the molded article continues to cool until the frozen layer is thick and rigid enough for molded article ejection. The pressure decreases as the molded article cools and the specific volume decreases. The outer frozen layer tries to maintain molded article dimensions while the core region continues to cool and shrink thus creating additional residual stresses in this known injection molding process. The greater the difference in temperature between the core and the surface, the higher the cooling stress.
Crystalline polymers also show the effects of flow-induced orientation. At temperatures above the melting temperature, a crystalline resin is amorphous. The resin injected at such temperatures contacts the cold mold surface and a thin oriented layer freezes forming some small crystalline regions. The crystals thus formed act as nucleating agents and because they are oriented, the crystals grown from them are also oriented. The intermediate and central core regions develop in much the same way as described for amorphous plastics.
Close tolerances are difficult to achieve in crystalline plastic molded articles formed by known injection molding processes because crystallization causes additional shrinkage and typical molding conditions cause large variations in crystallinity throughout the molded article. Crystal growth requires time and high mobility of molecules so that molecular segments can get close together and align themselves. V. M. Nadkarni et al., "Injection-Molding Semicrystalline Polymers", Plastics Engineering, August 1984 and S. Kenig et al., "Cooling Molded Parts--A Rigorous Analysis," SPE Journal, Vol. 26, 1970 describe how molding conditions influence crystallization. A high quench rate causes the onset of crystallization to start at a temperature below the crystalline melting temperature of the polymer. The crystallization onset temperature decreases with increasing quench rates. The rate of crystallization peaks over a narrow temperature range somewhere between the melting temperature and the glass transition temperature of the polymer. The specific heat of the resin increases significantly in this rapid crystallization temperature range. Ideally, the crystallization onset temperature should be near the temperature where the highest specific heat occurs in this temperature range. The resin should then cool slowly as it passes through the remainder of this temperature range, so that maximum crystallization occurs in the shortest period. Crystallization will also occur more readily under lower pressure because of increased molecular mobility. As discussed above, in known injection molding processes, the skin region and the core region cool at much different quenching rates which causes variable crystallization. The crystallization at the surface of the molded article will be less than at its center.
Several other problems are also inherent in the injection molding process. Molded articles with varying wall thicknesses will pack differently in different locations in the molded article. Overpacking occurs in areas of a molded article that fills too early and in a cavity of a family or multicavity mold that fill before the other cavities. Overpacking also occurs in the area near the gate wherein hot plastic is forced in with the relatively cool plastic. The overpacked areas are highly stressed and exhibit lower strength, warpage and hang-up in the mold. Common methods of reducing these problems include altering molded article wall thicknesses, relocating the gate, and profiling the injection pressure to reduce pressure during packing.
Differential densities, molecular orientations, thermal gradients, and crystallinity in plastic molded articles cause warping. In addition, weld lines are formed where two melt paths meet. If there are large enough temperature differences at the converging melt fronts, weak weld lines are formed. Conventional methods for minimizing this problem include adjusting gate locations, gate sizes or runner sizes. The molded article may even be redesigned to relocate the weld lines.
High precision and optical molded articles have additional molding problems due to the criticality of dimensional accuracy, homogeneity, and stability. It has been generally recognized in the literature (see, for example, page 34 of the second edition of "The Handbook of Plastic Optics", U.S. Precision Lens Inc., Cincinnati, Ohio) that all parts of a molded optic product must cool at the same rate during the cooling cycle to minimize molded article irregularity. To this end, molds have been built with heating and cooling passages for controlling cooling rate. Such sophisticated localized cooling and heating is costly, may be operator dependent and may not be uniformly effective.
Non-uniform stresses within a molded optical article also cause birefringence, which is the characteristic of having two indices of refraction with different values causing the separation of a light beam passing through the material into two diverging beams. Birefringence is a major problem in plastic molded optical articles such as lenses and optical discs. A linear relationship has been found to exist between orientation stress and birefringence. The greater the degree of orientation stress, the greater the birefringence. Both flow-induced orientation stress and stress caused by rapid cooling through the glass transition temperature with large temperature gradients, as in the known process of injection molding, cause birefringence.
The birefringence requirement for audio compact discs is 50 nanameters or less. Typical production discs exhibit birefringence of 20 to 30 nanameters. Similar optical disks used for computer data storage have even more stringent birefringence requirements. Shoji Ohsawa et al. "Thermosetting Resin Substrate For Computer Optical Memory Disk," 15 Nov. 1986/Vol. 25, No. 22 of Applied Optics make the following statements:
Birefringence is one of the most important properties of optical disks substrates when used for computer memory units, as this property is directly related to the carrier-to-noise ratio (CNR) and bit error rate. In particular, a magnetooptical disk using a delicate Kerr rotation angle as a signal requires a birefringence of less than 5 nm (double pass), and nearly 0 must be targeted. PA1 The principle stress difference defined here is caused by (1) a heterogeneous residue of mold pressure and (2) and heterogeneity of mold shrinkage. In general, the higher the mold pressure and the larger the mold shrinkage, the more heterogeneous becomes the residual stress. This results in a larger principal stress difference. It is, therefore, necessary to minimize the residual stress for a minimum birefringence.
U.S. Pat. No. 4,879,082 describes a method for molding polycarbonate resin optical discs to achieve essentially zero birefringence. In doing so, however, this method purposely molds stresses into the molded product which must later be removed by annealing for five hours at a temperature between 100.degree. C. and 120.degree. C.
L. I. Johnson, "Strain Free Injection Molding," Plastics Engineering (June 1963) states that heating a mold to a temperature well above the solidification temperature of the resin significantly reduces the residual stress in the molded article. Several methods have been proposed to heat a thin layer of material at the cavity surface during the time resin is being injected into the mold and then quickly cool the thin surface layer to minimize the molding cycle time. For example, U.S. Pat. No. 4,338,068 describes a method for controlling heating and cooling of a thin layer of material at the mold surface to cause thin and thick walls in the same plastic molded article to solidify at about the same rate. The method has the following steps:
Step (1) Heat a thin layer of material that forms the mold cavity surface using an external energy source such as electrical resistance heating, with the thin and thick wall locations of the molded article being heated independently.
Step (2) Close the mold.
Step (3) Inject resin into the mold until the cavity is filled.
Step (4) Using heating and cooling means and associated controls, controllably heat the mold cavity surface layer until the molded article is substantially free of either injection induced or secondary flow induced stresses.
Step (5) Use fast response cooling means such as volume controlled variable conductance heat pipes to cool the molded article rapidly with the thin and thick wall locations of the molded article being cooled at different rates that are also varied over time.
Step (6) When the molded article is rigid enough, open the mold and remove it.
Expensive additional equipment and complicated controls are required to heat and cool the surface of the mold at the proposed times and at the proposed rates. Complication and expense are increased further when various locations are heated and cooled independently from one another for molding parts with varying wall thickness.
European Patent Application 0 202 372 describes a method for heating and cooling a very thin metal cavity surface layer. Pressurized fluid is circulated behind a thin metal cavity surface layer to prevent distortion of the surface layer by balancing the pressure exerted by the resin melt when it is injected into the mold cavity. In other words, the pressurized fluid must provide rigid structural support for a thin metal cavity surface layer. The process has the following steps:
Step (1) Heat a thin layer of metal at the mold cavity surface until it is above the glass transition temperature of the resin to be molded by circulating hot fluid in the cavity behind a thin surface layer of metal.
Step (2) Close the mold.
Step (3) Inject resin melt into the mold cavity while continuously adjusting the pressure in the heating fluid to balance the changing pressure exerted by the resin melt on the thin metal wall.
Step (4) Discontinue circulating hot fluid that heated the thin metal mold cavity surface layer when the cavity is filled and continuously adjust fluid pressure as in Step (3).
Step (5) Circulate cooling fluid in the cavity behind the thin metal surface layer to cool the resin and continuously adjust fluid pressure as in Step (3).
Step (6) Open the mold and remove the article when the molded article is rigid enough.
The steps for heating and cooling the thin metal cavity surface layer in this method are similar to the method used in U.S. Pat. No. 4,338,068. In addition to having the same disadvantages, however, the method entails the complexity and problems associated with controlling the pressures in the circulating fluid so that it balances the pressure of the resin. The resin pressure changes suddenly and drastically at various times in the injection molding cycle such as at the instant when the mold is completely filled. Very sophisticated equipment is required to balance the pressure on both sides of the thin cavity wall during rapid changes of resin pressure.
U.S. Pat. No. 4,285,901 describes a mold for regulating the cooling speed of the molten resin injected or placed in a mold cavity. The molding method has the following steps:
Step (1) Bring the mold to operating temperatures.
Step (2) Close the mold.
Step (3) Inject or place hot resin melt into the mold cavity where a passive insulation layer located behind a thin metal surface layer causes the mold cavity surface temperature to increase rapidly to a temperature that prevents a solid layer of resin from forming during the time the resin is flowing into the mold.
Step (4) Cool the resin until the molded article is rigid enough to be removed from the mold.
Step (5) Open the mold and remove the molded article.
Steps (1), (2) and (5) of this method were previously known. In step (3), the mold cavity surface is provided by a thin metal layer and a layer of heat insulating material located between the surface layer and the mold housing. The thin metal layer and the heat insulating layer are intended to cause the thin metal surface layer to be heated by the molten resin immediately when the resin is injected or placed into the cavity in order to inhibit, to the greatest possible extent, the formation of the solid resin layer that would otherwise occur upon injection or placement of the molten resin in the cavity. This method had the aim of improving transfer of the molding surface onto the resin, precluding or minimizing weld and flow marks, and producing an article with uniform and reduced residual stress.
Upon analytical examination of the immediately above-described prior art mold and molding method, however, it becomes clear that whenever two materials at different temperatures suddenly come into intimate contact with one another, the temperature at the contact surfaces immediately change to a common temperature that is somewhere between the two initial temperatures. The common surface temperature is determined by the thermal conductivities, densities, specific heats, and initial temperatures of the two materials. The surface temperature then remains constant while the temperatures within the two materials change as heat flows from one to the other until the temperature at the far side of one of the materials starts to change. Thus, the surface temperature of the thin metal surface will increase immediately, but only to a temperature determined by the hot resin melt and the metal of the surface layer. The surface temperature will remain at this temperature as though the insulation layer did not exist until heat starts flowing through the metal surface layer into the insulation layer. The surface temperature will then increase further over a period of time as the insulator drags the metal surface temperature along as its own temperature increases. The mold surface temperature does not increase immediately to the higher temperature as it would if the insulator formed the mold cavity surface.
To prevent a solid resin layer from ever forming during injection of the resin, therefore, the mold described in U.S. Pat. No. 4,285,901 must be at a high initial temperature. For example, if the surface temperature of the mold cavity must rise immediately to 300.degree. F. so that it is slightly above the 295.degree. F. glass transition temperature of a low viscosity polycarbonate that is injected at 620.degree. F., the initial temperature of a nickel mold surface layer would have to be 289.degree. F. Because the solidification temperature of 295.degree. F. is only 6.degree. F. above the initial mold surface temperature, the cooling time to solidify the molded article would be excessive. The mold cavity surface is not raised above the flow stress relaxation temperature for an extended period, so that stresses can relax, or so that, within the resin rapid crystallization range, crystallization can occur at or near the surface of the molded article.
The frozen skin thickness increases with time, narrowing the flow path and thus increasing resin shear rate and shear stresses. To reduce shear stresses, the injection time for precision molded articles is often very short. Injection time for compact audio discs, for example, is typically 0.25 seconds.
For step (4) of the method in U.S. Pat. No. 4,285,901, the kind and thickness of the insulating layer is determined so that cooling of the resin is not hindered after the cavity has filled up. Therefore, while step (3) helps reduce the creation of flow stresses caused by formation of a solid resin skin during injection, steps (3) and (4) do not bring cavity surface temperatures high enough and then hold them until the shear flow stresses that do develop relax. In addition, faster cooling creates new stresses when the molten center shrinks as it solidifies within a solid resin skin. In the initial stage of research, the patentees state that they conducted experiments using a thermosetting resin to provide the mold surface. The thermosetting resin although effective for heat insulation, was softened or damaged when exposed to the hot molten resin. This is a serious deficiency making it unsuitable for a production mold that must be used many times an hour for injection, compression, or blow molding. They abandoned using an insulating layer at the mold surface as unserviceable and turned to using a thin metal layer to provide the cavity surface. The method used in making this mold involves (1) preparing a master mold in the shape of the article to be molded, (2) forming a thin metal layer on the master mold, (3) forming a layer of heat insulating filled thermoset material over the thin metal layer, (4) forming a backing member over the insulating layer and (5) removing the master mold.
An experimental method for minimizing residual stresses in molded parts is described by Liou in his 1987 M.I.T. Phd Thesis in which he used a 0.01 cm thick coating of Teflon-S as a mold cavity surface layer in an experiment to raise its temperature during the injection period. He reported a reduction of birefringence of 40% compared to a bare metal mold operating at the same temperature, but found the Teflon to be unserviceable. He concluded that in addition to the required thermal properties and thickness for the layer, the materials used for the layer should also satisfy the requirements of good wear resistance, high melting point, good bonding strength to the cavity and smooth surface finish but acknowledged that material for the layer which meets all the requirements and has a low mathematical product of thermal conductivity, specific heat and density is yet to be developed.
U.S. Pat. No. 4,340,551 describes a method which selectively heats a superficial layer of a mold cavity surface using high-frequency induction heating for the purpose of molding articles that are superior in surface gloss and exhibit no surface defects by forming a very thin pure resin surface layer to avoid dents and pits caused by filler materials. Induction heating of a thin iron rich surface layer to a temperature above the heat distortion temperature of the thermoplastic resin prior to injecting the resin into the mold was proposed to achieve this purpose. The conventional injection molding cycle is then used. This method cannot, however, substantially reduce the residual stresses in the molded article, particularly at or near the surface. A solid resin skin and shear flow stresses are created during injection and cannot relax because the mold surface temperature is below the glass transition temperature of the resin.
U.S. Pat. No. 4,054,679 describes a method for injection molding a parison that is later blow molded into biaxially oriented carbonated beverage bottles in which the full thickness of a core pin is kept at a temperature well above the solidification temperature of the resin throughout the injection molding process. The injection molding portion of the process purposely creates thermal and stress gradients in the parison wall to compensate for the differences in stresses between the inner and the outer surfaces of the parison wall that are caused by the subsequent blow molding operation, which stretches the parison wall. The core pin that defines the inner surface of the parison is maintained at a temperature well above the resin solidification temperature throughout the injection molding process while the outer surface is cooled and solidified. When the outer surface is rigid enough for the parison to be handled, the parison is removed from the mold. The inner surface is still fluid or semi-fluid when the parison is removed. This is a very specialized process that purposely creates stress gradients in the molded article and is not suitable for producing high precision or optical molded articles where the goals include producing homogeneous molded articles with minimum residual stresses.
Methods for molding precision optical lenses have also been proposed. For example, U.S. Pat. No. 4,364,878 describes a method for molding optical lenses that combines injection and compression molding according to the following steps:
Step (1) Heat die inserts to or slightly below the glass transition temperature of the resin.
Step (2) Close the mold with the dies separated so that the mold cavity volume is larger than the volume of the finished lens.
Step (3) Inject a mass of resin equal to the mass of the finished lens into the oversized cavity using low pressure.
Step (4) Press the dies together to coin the resin mass to completely fill the resulting cavity.
Step (5) Controllably cool the lens so that all points in the lens cool to the glass transition temperature substantially at the same time.
Step (6) Open the mold and remove the lens.
Because the dies in the above-described method are at or slightly below the glass transition temperature of the resin, flow stresses will be created and cannot be relaxed. The mechanism used to controllably cool the lens is the insertion of plugs of different thermal conductivities into the dies. This will provide somewhat more even cooling between thinner and thicker locations of the lens; however, the surface of the lens will solidify long before the center. Because the mold cavity surface temperature is so close to the solidification temperature of the resin when the resin is injected, the temperature gradients through the lens thickness are very large at the onset of solidification. This creates additional residual stresses as the solidifying center region shrinks within the already solidified surface of the molded article. The coining step also requires additional moving parts, including a hydraulic actuator and hydraulic circuit.
U.S. Pat. No. 4,836,960 describes a combination of injection and compression molding in the following steps:
Step (1) Heat mold dies well above the glass transition temperature of the resin.
Step (2) Inject resin melt into the mold.
Step (3) Bring dies together to force excess resin out of dies.
Step (4) Slide both dies together inside a mold sleeve to close the resin entrance port.
Step (5) Cool the lens to the glass transition temperature.
Step (6) Move the mold to another station for further cooling.
Step (7) Remove dies from sleeve and allow the lens to cool until it shrinks free of the mold sleeve.
This process requires several work stations, a great amount of handling, and excessive cooling time. The cooling time is due to the need to cool the entire mass of the mold dies and is much longer than is required for the resin to relax. It takes ten minutes to cool to the glass transition temperature before the mold is moved to the next station where it cools further until it can be handled.
The injection/compression molding method is also common in optical disc molding. By injecting the molten resin into an oversized cavity, the flow path is widened, thereby reducing the shear rate and thus shear stresses in the resin. After the cavity is filled, pressure is maintained on the cavity dies, so that they compress the resin and follow it as it shrinks. Although this method does reduce flow stresses somewhat, the cold cavity walls cause a solidified or frozen skin to form immediately and creates significant stresses that are acceptable for audio discs but are not acceptable for rewritable data storage discs.